1. Field of the Invention
The present invention relates to a semiconductor substrate having: a first semiconductor layer; an oxide layer which is formed on the first semiconductor layer; and a second semiconductor layer which is formed on the oxide layer.
2. Background Art
A backside illumination image pickup device has been proposed in which an image is taken while the backside of a semiconductor substrate is irradiated with light, charges that are generated in the semiconductor substrate in accordance with the light are accumulated in a charge accumulation region formed in a surface side of the semiconductor substrate, and a signal corresponding to charges accumulated in the region is output to the outside by a CCD, a CMOS circuit, or the like that is formed in the surface side of the semiconductor substrate. Hereinafter, a usual pickup device which is currently widely used is referred to as a surface illumination image pickup device, in contrast to the backside illumination image pickup device.
In a backside illumination image pickup device, in order to enable a color image to be taken, a structure of a color filter, microlenses, or the like must be formed in the back face side of a semiconductor substrate in accordance with a structure formed in the surface side of the semiconductor substrate. Therefore, it is necessary to dispose an alignment mark for positionally aligning the structure formed in the surface side of the semiconductor substrate with that formed in the back face side.
JP-A-2003-273343 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) discloses a system in which, a mark is formed by a device constituting layer on the surface side of a backside illumination image pickup device, and the mark is detected by red light or IR light from the back face side. In this method, however, there are disadvantages such as that the wavelength of the detection light is long and hence the alignment accuracy is lowered, and that, when a silicon layer for performing photoelectric conversion is sufficiently thick, visible light is not substantially transmitted through the silicon layer, and therefore the intensity of light reflected from the mark is significantly lowered and the detection becomes uncertain.
As an alignment mark which can be surely detected from both the front and back sides, JP-A-2005-150463 proposes an alignment mark which is configured by forming a trench penetrating through a silicon layer, and embedding an oxide film in the trench, and JP-A-2005-268738 proposes an alignment mark which is configured by forming a trench penetrating through a silicon layer, and embedding a material different in quality from silicon, in the trench.
In the alignment marks proposed in JP-A-2005-150463 and JP-A-2005-268738, however, there occur the following problems.
It has been experimentally known that, in a backside illumination image pickup device, a silicon substrate (photoelectric converting region) must have a thickness of about 10 μm in order to absorb most of visible light. For example, a usual automatic alignment mark is configured by squares of 4 μm which are arranged in 7× several rows at intervals of 4 μm. In the case where the silicon substrate has a thickness of 10 μm, in order to embed an oxide film or a material different from silicon in a trench of 4 μm square, a film of an embedding material must be formed at a thickness which is larger than 2 μm, and the material of the same film thickness must be removed from the substrate. Considerable loads are applied to these steps, and hence there arises problems such as that the production cost is increased.
In the case of a backside illumination image pickup device, the production steps include a heating step. When a high-temperature thermal process is executed on an alignment mark formed by proximately embedding a material which is different in thermal expansion coefficient from silicon, in a column-like shape, there is a danger that the mark structure is broken by thermal stress during the high-temperature thermal process. In the case where polysilicon or amorphous silicon is embedded in a trench, such thermal stress is reduced. In this case, however, the refractive index is equal to that of silicon, and hence it is difficult to detect the mark.